The invention relates to a method for the electroless deposition of a metal layer from a metallization liquid comprising a metal compound in solution on a flat side of an article to be metallised, for example, a flat disc, more in particular a master disc.
The invention also relates to a device for carrying out the method and a master disc obtained according to the method.
A method of manufacturing a metal matrix is described in U.S. Pat. No. 4,931,147 (=PHN 12 671, herewith incorporated by reference). A glass disc is provided with a photoresist layer in which an information track is provided. A nickel layer is provided on the master disc thus formed in an electroless nickel-plating bath. A further metal layer is provided on the nickel layer by electrodeposition, which metal layer is then separated from the master disc as a metal shell in which the information track of the photoresist layer is copied. The metal shell may then be used as a matrix for manufacturing optically readable discs. The method according to the invention is suitable for the electroless deposition of the nickel layer on the master disc, as well as the electroless deposition of the metal layer on other flat articles.
In this U.S. patent a master disc is disclosed which consists of a flat polished glass disc which on one side is provided with a layer of a usually positively acting photoresist. A suitable photoresist is, for example, a resist based on novolak and orthonaphthoquinone diazide. The photoresist layer is exposed in the form of a pattern, to, for example, modulated laser light. As a result of which the exposed parts become soluble in a basic solution of, for example, NaOH in water. In order to improve the bonding between the glass disc and the photoresist, a bonding layer is provided on the glass disc before the photoresist layer is provided. A suitable bonding layer is, for example, titanium acetyl acetonate. In order to obtain a readily bonding nickel layer, this U.S. Patent describes a number of measures. For example, before providing the nickel layer the top side of the master disc may be treated with a detergent and a solution of aminosilane. It is further suggested to provide the metallization liquid with sodium benzene disulphonate to reduce stresses in the deposited metal layer. After the treatment with aminosilane the top side of the master disc may be treated with tannin. This has a favourable effect on the bonding of the nickel layer. It is stated that the various types of liquids may be sprayed, atomized poured, etc. on the surface of the master disc and that dipping the master disc in the various solutions is also possible. Further data with respect to the electroless nickel-plating of master discs may be derived from this U.S. Patent.
It is known from GB-A 1,058,021 to silver-plate flat articles by spraying the article with liquids. A separate nozzle is used for each liquid, the various nozzles are united to form one assembly of movable spraying device which is controlled by means of suitable control means in such a manner that the nozzle to be used is always opposite to the article. The nozzles and also the article are in a closed metallization device. The article, for example, a master disc, is provided on a rotating spindle in an inclined position. As a result of the inclined position the sprayed liquid constantly flows off the disc. The uniformity of the sprayed liquid film is promoted by the rotation. So these measures together ensure a uniform film of constantly refreshed liquid to be created on the top side of the article.
The method known from the said GB-A 1,058,021 is not suitable for the electroless metallization of articles by means of a stable metallization liquid, for example, a stable nickel plating liquid. Nickel salts and reduction agent, for example, are present side by side in a stable nickel-plating liquid without reacting with each other. Only the presence of a catalyst at the surface of the article to be metallised leads to the deposition of metal from the liquid. For the deposition of nickel, for example, palladium may serve as a catalyst. When using stable metallization liquids the kinetics of the metallization plays an important part. The process of depositing metal on a surface from a stable metallization liquid, for example, a nickel-plating liquid comprising nickel sulphate complexed with pyrophosphate and ammonium, is kinetically and thermodynamically limited. This means that a minimum temperature is necessary and that movement of the metallization liquid should be limited to such an extent that the deposition of metal is not prevented or inhibited by excessive kinetic supply of ions (hydrogen ions). The method of this British Patent Specification is excellently intended for use of a non-kinetically limited metallization. Furthermore a disadvantage of the known method is the considerable consumption of process liquids. The liquids at the surface of the article are constantly refreshed, which in a non-kinetically limited metallization, leads to a rapid metallization. However, it does have for its result that the liquids used are used effectively only partly.